Shahinyan, Karlen2020-02-262020-02-262017-12https://hdl.handle.net/11299/211798University of Minnesota Ph.D. dissertation. December 2017. Major: Astrophysics. Advisor: Lucy Fortson. 1 computer file (PDF); ix, 160 pages.Blazars are active galactic nuclei with jets aimed in our direction. They are one of the most energetic astrophysical phenomena and in the very-high-energy (VHE; > 100 GeV) gamma-ray regime comprise the dominant extragalactic source class. Variability in the non-thermal emission of blazars is one of the key observables for discerning the details of their emission regions. In this thesis, I explore VHE gamma-ray emission properties of blazars in a variety of states. After a detailed description of the VERITAS instrument and data analysis, I present an in-depth study of an unusually stable extreme blazar (formerly, an unidentified source), HESS J1943+213, for which no concrete evidence of variability has been detected to date. The study includes the steps towards identifying HESS J1943+213 as a blazar using VLBA observations and estimation of the source redshift with gamma-ray spectra. Models of leptonic emission from the source and hadronic emission from cosmic-ray interactions along the line of sight are presented for explaining the gamma-ray observations. I then proceed to characterize the emission of two blazars in rapidly flaring states, Mrk 421 and MS 1221.8+2452. The dataset for Mrk 421, the first blazar detected in VHE gamma rays, comes from one of the brightest VHE gamma-ray flares ever observed and offers a unique opportunity for exploring VHE gamma-ray and multi-wavelength emissions of the source on timescales of few minutes. The flare of MS 1221.8+2452, on the other hand, allows for the construction of the first VHE gamma-ray spectrum of the source, which is used along with contemporaneous multi-wavelength observations to construct and model the SED of the source during the flare. Moreover, the source exhibits a spectral hysteresis pattern in X-rays during the flaring period, which is tied to the acceleration and cooling timescales of particles in the emission zone. Finally, I investigate, PG 1553+113, a blazar exhibiting long-term periodic flux modulations in lower-energy gamma rays – due to periodic accretion flow instabilities, jet precession, or the presence of a binary supermassive black hole system – and search for a similar periodicity signal in the VHE gamma rays.enblazarsvariabilityvhe gamma raysThesis or Dissertation